Encoders for quadraphonic sound system

ABSTRACT

Methods and apparatus for combining a plurality of channels of audio program information into two composite signals suitable for recording or transmission on a medium having only two independent tracks, and for presentation over the two loudspeakers of stereophonic playback apparatus which at the same time will admit of decoding into a plurality (e.g., four) separate signals corresponding to the originally encoded signals for presentation over corresponding loudspeakers. The two composite signals respectively contain input signals intended for presentation on loudspeakers positioned at the left front and right front corners of a listening area, and both composite signals contain, at reduced amplitude, signals intended for presentation on loudspeakers positioned at the left back and right back corners of the listening area. The encoder includes all-pass phaseshifting networks designed to transmit all frequencies in the frequency range of interest which are operative to cause the left back and right back components of one composite signal to be in quadrature relationship with corresponding components in the other composite signal.

United States Patent 1 Bauer 1 June 17, 1975 1 ENCODERS FOR QUADRAPHONICSOUND SYSTEM [75] Inventor: Benjamin B. Bauer, Stamford,

Related US. Application Data [63] Continuation-in-part of Ser. No.328,814, Feb. 1, 1973, abandoned, which is a continuation-in-part ofSer. No. 251,544, April 21, 1972, abandoned, Continuation of Ser. Nos,44,224, June 8, 1970, abandoned, Ser, No. 124,135, March 15, 1971, Pat.No. 3,821,471, and Ser. No. 288,829, Sept. 13, 1972,

OTHER PUBLICATIONS The Compatible Stereo-Quadraphonic SQ" Record, Bauer,Audio Magazine, Oct. 1971.

The Sansui 08 System, Audio Magazine, Oct. 1971. Discrete vs. SQ MatrixQuadraphonic Disc Bauer, Audio Magazine, July 1972.

The Joke System by Michael Gerzon, Hi Fi News, June 1970, pp. 843, 847.

Primary Examiner-William C. Cooper Assistant Examiner-Thomas D'AmicoAttorney, Agent, or Firm-Spencer E. Olson [57] ABSTRACT Methods andapparatus for combining a plurality of channels of audio programinformation into two composite signals suitable for recording ortransmission on a medium having only two independent tracks, and forpresentation over the two loudspeakers of stereophonic playbackapparatus which at the same time will admit of decoding into a plurality(e.g., four) separate signals corresponding to the originally encodedsignals for presentation over corresponding loudspeakers. The twocomposite signals respectively contain input signals intended forpresentation on loudspeakers positioned at the left front and rightfront corners of a listening area, and both composite signals contain,at reduced amplitude, signals intended for presentation on loudspeakerspositioned at the left back and right back corners of the listeningarea. The encoder includes all-pass phase-shifting networks designed totransmit all frequencies in the frequency range of interest which areoperative to cause the left back and right back components of onecomposite signal to be in quadrature relationship with correspondingcomponents in the other composite signal.

33 Claims, 37 Drawing Figures SHEET TNP I PATENTEDJuu 17 ms 1 1 O ShEET6 .707R (L L,

234 Mr) (H Jon (R 240 ENCODERS FOR QUADRAPHONIC SOUND SYSTEM This is acontinuatiomin-part of now abandoned application Ser. No. 328,814 filedon Feb. 1, 1973 in the name of Benjamin B. Bauer (which, in turn, is acontinuation-in-part of now abandoned application Ser. No. 251,544 filedon Apr. 21, 1972 as a continuation of now abandoned application Ser. No.44,224 filed June 8, 1970, and of application Ser. No. 124,135 filed onMar. 15, 1971, now U.S. Pat. No. 3,821,471) and of now abandonedapplication Ser. No. 288,829 filed on Sept. 13, 1972 in the name ofBenjamin B. Bauer.

CROSS-REFERENCE TO OTHER APPLICATIONS This invention is related to thesubject matter of the following other co-pending applications, all ofwhich are assigned to the assignee of the present invention andapplication: Ser. Nov 164,675 filed July 21, 1971 as acontinuation-in-part of now abandoned application Ser. No. 40,510 filedMay 26, 1970 now also aban' doned; Ser. No. 44,196 filed June 8, 1970,now U.S. Pat. No. 3,708,631; Ser. No. 271,470 filed July 13, 1972 as adivision of application Ser. No. 44,196, filed June 8, 1970, now U.S.Pat. No. 3,794,780; Ser. No. 185,050, filed Sept. 30, 1971 as a divisionof now abandoned application Ser. No. 44,224 filed June 8, 1970, nowU.S. Pat. No. 3,813,494; Ser. No. 251,636 filed May 8, 1972 as acontinuation of now abandoned Ser. No. 81,858 filed Oct. 19, 1970, nowU.S. Pat. No. 3,812,295; and Ser. No. 243,800 filed Apr. 13, 1972, nowPat. No. 3,761,628, Ser, No. 118,271 filed Feb. 24, 1971, now U.S. Pat.No. 3,784,744.

BACKGROUND OF THE INVENTION There is an increasing interest inmultiple-channel recording and reproduction because of the variety ofsounds and music forms that can be achieved thereby. 1n the early daysof phonograph, only single channel or monophonic recording was used, andas early as 40 to 50 years ago, investigators realized the value ofrecording and transmitting two separate channels of information, whichin modern parlance is known as binaural or stereophonic sound. However,even two channels of information are considered insufficient for fullillusion of reality. For example, when a listener is placed in front ofa symphony orchestra he hears sounds arriving from many differentdirections and from a variety of instruments, as well as reflectionsfrom the walls and ceiling, which gives him an accustomed illusion ofspace perspective. However, when reproduction is accomplished byutilizing only two channels it is difficult, if not impossible, toproduce true reality with respect to spatial perspective. Earlyexperiments have demonstrated that a minimum of three independentchannels are needed to convey a satisfactory illusion of reality in thereproduction of orchestral music.

The modern stereophonic phonograph is capable of recording, or encoding,modulation along two separate channels, which geometrically are at 90 toeach other and at 45 to the disc surface. It is usual practice toinclude a third, or center, channel by matrixing or combining it as anin phase phantom channel to the other two, which causes it to berecorded as lateral modulation parallel to the record surface. Uponreproduction, the third (or central) channel appears on the twoloudspeakers of the stereophonic phonograph with equal loudness andin-phase relationship and an observer placed centrally between theloudspeakers perceives the illustion of the third channel being locatedbetween the other two. Although there have been attempts to reproducethe third or center channel on a separate loudspeaker, the results havenot been entirely satisfactory, and most stereophonic systems, eventhough many stereo records carry a center channel, employ only twoloudspeakers.

In the aforementioned co-pending application Ser. No. 164,675 of WilliamS. Bachman there is described a system for providing third and fourthplayback channels to otherwise two-channel systems by feeding third andfourth loudspeakers with signals respectively rep resenting the sum anddifference between the left and right channel signals. The left andright loudspeakers may be located, for example, on opposite sides of alistening area, with the loudspeakers for the two virtual channelspositioned at opposite ends of the listening area. Each loudspeakerdisplays the particular information fed to its channel accompanied byhalf-power sig nals from its adjacent channels. This system provides apseudo-four-channel effect, but does not give a complete illustion ofeach channel appearing independently on its corresponding loudspeaker.

A better illusion of each channel appearing independently on itscorresponding loudspeaker is provided by the system described in theabove-mentioned U.S. Pat. No. 3,708,631 which includes four gain controlamplifiers through which the four separate channels of information arerespectively applied to corresponding loudspeakers, and a logic controlcircuit which derives its signals from the left and right outputterminals of the transducer for automatically controlling the gaincontrol amplifiers to enhance the realism of four separate channels ofinformation. While this system provides a significant improvement in theart of reproduction of recorded sound, it has a number of drawbacks asfollows: (1) The system provides for both a single back channel and theinformation originating from this back channel, which is encoded as adifference signal or as a vertically oriented elliptical signal, haslittle or no component in the lateral or sum direction, and accordingly,as the record is played on a monophonic phonograph or transmitted over amonophonic radio station, the signal identified with the back directionis greatly attenuated or disappears altogether; (2) In the case of astereophonic disc record, it is undesir able to apply informationoriginating from the back in the vertical direction because it tends tomake cutting and pressing of the record more difficult; (3) When astereophonic disc record carrying back information as verticalmodulation is played on a conventional stereophonic player, the signalscorresponding to the back direction appear at the two loudspeakersout-of-phase, or significantly so, thereby causing a relativelyunpleasant pressure in the ears" sensation; (4) 1n conventionalstereophonic practice the two loudspeakers are normally placed in twoadjacent corners of the listening room, and it is conventional in theproduction of fourchannel recordings to have the four sources originatefrom the four corners of the room or listening area. However, thesystems described in the aforementioned co-pending applications aredesigned to preserve symmetry withh loudspeakers placed centrally of thefour walls of the listening room. If the loudspeakers were placed in thecorners, the aspect of the originally recorded sound would be shifted by45, causing an inconsistency confusing to the listener. Also, sincethere are practical difficulties in finding suitable locations forloudspeakers centrally of the walls in most homes, it is preferable thatthe reproducing system permit the placement of the loudspeakers at thecorners of the listening room.

SUMMARY OF THE INVENTION A principal object of the present invention isto provide methods and apparatus for combining four channels of programinformation, edited for presentation on four loudspeakers placed at thecorners of a listening area, into two composite signals suitable forpresentation over the two loudspeakers of stereophonic sound reproducingapparatus which at the same time will admit of decoding into fourseparate signals corresponding to the four signals originally encoded,for pre sentation over four corresponding sound-reproducing devices.

Briefly, the foregoing object is obtained by combining the four channelsof information for convenience identified as L, for left front". R,frright front", L,, for left back" and R for right back", to form twocomposite signals L (left total] and R (right total) for recording ortransmission on a two-channel medium, such as a stereophonic disc recordor a two-track magnetic tape. The encoding apparatus contains all-passphaseshift networks designed to transmit all frequencies from about toabout 20,000 Hz and summing networks interconnected in a manner suchthat upon linear matrixing of four equal-amplitude input signalsdesignated L,, R,, L,, and R the L and R composite signals have thefollowing characteristics:

1 The L, signal appears only in the L composite signal and the R; signalappears only in the R composite signal; thus the front signals arecompletely isolated from each other. When L, and R are applied to a stereophonic cutter, L, produces a 45 modulation, while R; produces a 45modulation, precisely as with conventional stereo.

2. The left back (L signal appears in both the L and R composite signalsat reduced amplitude, 0.707 L in the preferred embodiment, and inquadrature with each other, with the L component in the L compositesignal preferably leading the corresponding signal in the R signal. Thiscauses a stereophonic cutter stylus to describe a circular motion in theclockwise direction, which when combined with the lengthwise motion ofthe groove becomes a clockwise helix.

3. The right back (R signal also appears in both the L and R compositesignals at reduced amplitude, 0.707 R in the preferred embodiment, andin quadratuure with each other. The R component in the R sig nal leadsthe corresponding component in the L signal which causes the cutterstylus to describe a circular motion in the counterclockwise direction.

The front channel modulations are fully equivalent to the correspondingstereophonic modulations and provide the full channel separation onplayback of which the pickup is capable, while the back channelmodulations are of a character to cause the pickup stylus to describe acircular motion in one direction for a left back signal and in theopposite direction for a right back signal.

BRIEF DESCRIPTION OF THE DRAWING An understanding of the foregoing andadditional aspects of this invention may be gained from consideration ofthe following detailed description, taken in conjunction with theaccompanying drawing, in which:

FIG. I is a schematic diagram of a prior system for recording fourchannels of information on a stereophonic record;

FIG. 2 is a vector diagram useful in explaining the motion of the cutterstylus in response to application of left, right, center and differencesignals;

FIG. 3 is a cross-sectional view of a fragmentary portion of a recordshowing four record grooves on a greatly enlarged scale, to illustratethe motion of the cutter in response to various signals;

FIG. 4 is a schematic diagram of a prior art stereo phonic playbacksystem for providing the illusion of a third channel;

FIG. 5 is a schematic diagram ofthe system described in theaforementioned US. Pat. No. 3,708,631 for recording four channels on atwo-track stereophonic record;

FIG. 6 is a greatly enlarged illustration of a record grooveillustrating the effect of applying the difference" signal to the leftand right channels through a phase-shift network;

FIG. 7 is a schematic diagram of one embodiment of an encoder accordingto the invention for combining four independent signals intended forultimate presentation on four separate loudspeakers;

FIG. 8 is a vector diagram useful in explaining the operation of theencoder of FIG. 7;

FIG. 9 is a plan view ofa listening area illustrating the location offour loudspeakers therein and the phasor diagrams of the signalsappearing on the four loudspeakers;

FIG. 10 is a pair of phasor diagrams useful in explaining the principlesof the invention;

FIG. 11 is a schematic diagram of another encoder according to theinvention;

FIG. 12 is a schematic diagram of decoder apparatus described in US.Pat. No. 3,784,744, useful in explaining the efficacy of the encoder ofFIG. 11;

FIG. 13 is a schematic diagram of an alternative form of encodingapparatus embodying the invention;

FIG. I4 is a schematic diagram of still another alternative form ofencoding apparatus embodying the invention;

FIG. 15 is a diagram similar to FIG. 8 illustrating the motion of thecutting or playback stylus of stereophonic recording or reproducingapparatus in response to signals encoded in accordance with theinvention;

FIGS. 16 and 17 are phasor diagrams useful in explaining the operationand advantages of the encoder of FIG. 14;

FIG. 18 is a schematic diagram of a modified form of the encoder of FIG.14, and FIGS. 18A, [88, and 18C are diagrams illustrating the operationthereof;

FIG. 19 is a schematic diagram of an encoder described in applicantsco-pending US. Pat. No. 3,761,628, and FIGS. 19A, 19B and 19C arediagrams illustrating the operation thereof;

FIG. 20 is a schematic diagram of still another alternative form ofencoding apparatus, and FIGS. 20A, 20B, and 20C and 20D are diagramsuseful in explaining its operation;

FIG. 21 is a schematic diagram of still another encoder, and FIGS. 21A,21B, and ZIC are diagrams useful in explaining its operation; and

FIG. 22 is a schematic diagram of encoding apparatus embodying thefeatures of the three encoders illustrated in FIGS. l8, l9 and 20, andFIGS. 22A and 22B are phasor diagrams useful in explaining itsoperation.

DISCUSSION OF THE PRIOR ART By way of background for betterunderstanding the present invention, the current method of recordingstereophonic signals including a third or center channel, and a methodof reproducing the signals over a stereophonic two-loudspeaker systemwill be described with reference to FIGS. 1-4. The currently providedleft (L), right (R) and center (C) signals are applied to the twoterminals of a stereophonic cutter 10 having a cutting stylus ]2 whichis adapted to cut a groove in the lacquer of a master disc 14, revolvingon a recording turntable (not shown). The C signal is supplied through asignal splitter I6 of known configuration resulting in application ofequal portions thereof, equivalent to 0707C, to each of the L and Rterminals in-phase. AS is well known in the groove cutting art, the tipof the cutter is capable of motions contained within a surface generallyperpendicular to the disc in the manner portrayed by the vector diagramof FIG. 2. When a left signal L is applied, the stylus executes motionsalong the arrow L, which is at an angle of 45 to the horizontal, andwhen an R signal is applied, the stylus motion is along the arrow R, atan angle of 45 to the horizontal. Application of 0707C to each of the Land R terminals in-phase causes motion of the stylus along the arrow C,equal in magnitude to 0.707 (L i-R), which is of the same magnitude aseither L or R, but directed horizontally. It will be appreciated thatinstead of applying the L, R and C signals directly to the cutter, asshown in FIG. 1, they may, in keeping with common practice, first berecorded on a two-track master tape recorder and the output of the tapereproducer used to drive the record cutter. Discussion of the differencesignal D illustrated in FIGS. 1 and 2 will be deferred until later.

The type of groove modulation resulting from the just-describedprocedure is shown in FIG. 3. When only the left signal L is applied,the groove is modulated in accordance with the arrow L, which isessentially confined to one wall of the groove. Similarly, when the Rsignal is applied, the modulation is in the opposite wall of the groovein the direction of the arrow R, which, it will be noted, isperpendicular to the arrow L. Application of equal amounts of the centersignal C to the L and R lines causes both walls of the groove to besimultaneously and equally modulated in the directions indicated by thearrows L=0.707C and R=0.707C, resulting in horizontal or side-to-sidetranslation indicated by arrow C.

Apparatus for reproducing a stereophonic record carrying L, R and Csignals recorded in this manner, schematically illustrated in FIG. 4,includes a stereophonic pickup having a cartridge 18 and a stylus 20which enters the groove in the record and is actuated by the groovemodulations to deliver output voltages on the L and R terminals. If onlyL signal modulation is present in the groove. an output signal appearsonly at the L terminal and is amplified by a suitable power amplifier 22and reproduced by a loudspeaker 24. Similarly. when only R signalmodulation is present in the groove, an output voltage appears at onlythe R terminal of the pickup, which is amplified by power amplifier 26and applied to its respective loudspeaker 28. When the groove haslateral modulation consisting of the presence of equal amounts of leftand right signal, then equal signals, namely, 0.707C, appear at both theleft and right loudspeakers, resulting in the appearance of a phantomsource C (shown surrounded by a dashed line circle) midway betweenloudspeakers 24 and 28. However, this illusion is preceptible only tothe centrally located observer 30; when he moves to either side, the Csignal is heard over the nearest loudspeaker unless special precautionsare made to adjust the directional characteristics of the loudspeakerswith respect to the position of the observer.

It will be noted that the described three-channel record is compatiblebecause the L, R and C signals all have a horizontal component and thuswill be heard when played on a monophonic player, which is sensitiveonly to lateral modulation, albeit their relative intensities will notbe in the exact balance initially intended by the recording directorsince the horizontal components of L and R are 0.707 of C. In reality,in spite of the introduction of a third channel, the abovedescribedsystem reproduces only two independent channels of information. Thethird channel, C, is contained in both the left and right channels andthe listener will, therefore, usually hear it reproduced from theloudspeaker nearest to him. This center channel may be presented on aseparate loudspeaker system, as shown in dotted lines in FIG. 4, andamplifiers are commercially available for this purpose. This permits theobserver to percieve the center information without having to locatehimself equidistant from the left and right speakers, although such acenter channel loudspeaker tends to cause the sounds of the other twoloudspeakers to appear to be pulled in toward the center.

Reverting to FIGS. 1-3, a fourth channel, D, may be introduced to thetwo-channel stereophonic system by dividing it into equal parts by asignal splitter 32 and applying them in phase-opposition to the left andright channels. As shown in FIG. 2, application of the D signal in thismanner causes motion of the stylus in the vertical direction, along thearrow D, to an extent specified as 0.707 times the amount of D containedin the left and right channels subtracted from each other; i.e., 0.707(L-R). As seen in FIG. 3, this causes the left and right motions of thestylus to be out-of-phase relative to each other, resulting in up anddown motion. When vertical modulation is reproduced by the system ofFIG. 4, the loudspeaker cones are driven in opposite directions,resulting in out-of-phase sound pressures applied to the ears of thelistener, and since this condition of pressure on the ears does notcorrespond to any known normal listening experience, the observer isunable to localize the sound. The difference signal D appears at someindefinite point in space, shown as D in a dashed circle, and thelistener is unable to locate its whereabouts. Furthermore, somelisteners of such outof-phase sound have complained of a peculiarpressure in the ears sensation. This is in part overcome, however, bythe system described in the aforementioned Bachman application Ser. No.l64,675 wherein the difference signal, as well as the center signal, arereproduced on separate loudspeakers.

To afford better compatibility with monophonic and conventionalstereophonic players, while at the same time improving the illusion offour separate channels during playback, the difference signal D may beapplied in the manner suggested in applicants article en titled SomeTechniques Toward Better Stereophonic Perspective", IEEE'IRANSZ-K'TION'S OF AUDIO. Vol. ALl-l I, No, 3, lvlaydune, I963. Inkeeping therewith, and as is illustrated in FIG, 5, instead of applyingthe difference signal equally and oppositely to the left and rightchannels as in the circuit of FIG. I, the D signal is applied through anacoustical phase shift network 32 which splits the incoming signal intotwo equal amplitude signals D, and D each containing all ofthefrequencies of the D signal, but displaced in phase with re spect toeach other. Relative phase displacements in the range of l l() to l70have been successfully used, with an angle of l35 being particularlysuitable. It can be readily demonstrated that when the two signals arethus displaced relative to each other. the tip of the stylus instead ofundergoing a purely up and down motion as shown in FIG. 3, executes theelliptical motion illustrated in FIG. 6. The limits of stylus motion areshown by the dashed lines and the direction of motion of the ellipsedepends on whether D leads D or vice versa. The important considerationis that the groove has a horizontal component defined by the horizontalwidth of the ellipse, whereby both monophonic and stereophonicphonographs will reproduce all four signals; that is, the record withfour separate channels will be fully compatible with the older playbacksystems, albeit with monophonic systems the signal D is attenuated byabout 8 db.

The realism of reproduction of four separate Chtlt'l nels of informationrecorded as described above is enhanced by the control system describedin the aforementioned US. Pat, No. 3.708,63l which derives signals fromthe left and right terminals ofthe transducer, separates the compositesignals into their respective components, compares the magnitudes ofthese components, and actuates gain control amplifiers in the respectiveloudspeaker circuits in concert with the loud ness of the respectivecomponents in a manner to give a realistic illustion of four separateindependent sources of sound.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 7, thereis shown in schematic form a first embodiment of an encoding ormatrixing system for combining four independent signals, intended forultimate display on four separate loudspeakers, into two compositesignals for recording or trans mission on a two-track medium, such astereophonic disc record or a two track tape. The encoder includes fourinput terminals 40, 42, 44 and 46 for receiving four separate inputsignals which, for convenience, will be designated left front (L leftback (L right back (R and right front (R respectively. Thesedesignations signify the locations in a listening area of the fourloudspeakers on which the signals are intended for ultimatepresentation. These signals are identified by vertical arrows of equallength which signify, for purposes of the analysis to follow, that theincoming signals are assumed to be of equal magnitude and referred tothe same phase reference. The encoding matrix includes six alLp-assphaseshift networks 48, 50, 52, S4, 56 and 58 designed to introduce asubstantially constant phase shift to the applied signal over thefrequency range of interest without altering their magnitudes. Each ofthe networks has a reference phase shift d1, which is a function offrequency, the two phase shifters and 56 introducing only the referencephase shift, Phase shifters 48 and 58 provide a phase shift equal to(il:+45), and networks 52 and 54 provide a phase shift equal tolil1+9tll It is to he noted that according to the convention used hereinthe phasc shift angles produced by phase-shifters 48-58 are laggingangles. In other words, a network with phase-shift of (ill-+90")produces an out put lagging 90 behind that produced by a network withphase shift (with).

The signals L, and R,, respectively identified with the left front andright front loudspeakers are applied via their respective terminals 40and 46 through their asso ciated all-pass networks 48 and S8 torespective summing circuits and 62. The left back signal L,, is ap pliedto both of phase shift networks 50 and 52, the output signal from theformer being applied to summing circuit 60 with attenuationcorresponding to the multiplicand 0.707, and the output signal fromnetwork 52 is applied to summing circuit 62 with the same attenuation.The right back signal R,-, is similarly applied to both of phase-shiftnetworks 54 and 56, the output signals from which are respectivelyapplied with 0,707 attenuation to summing circuits 60 and 62. Thesuinming circuits 60 and 62, which are of conventional design and wellknown to ones skilled in the art, are operative to produce respectivecomposite signals L and R at their corresponding output terminals 64 and66. These signals may be applied to the left and right termi nals of astcreophonic disc record cutter, for example, or to the two recordingheads of a two track tape recording apparatus, or to any other knowntwo'track medium, in a manner which will be apparent to ones skilled inthe art.

Although the encoding apparatus has been thus far described in terms offour input signals, if it is desired to have a signal appear centrallyin the reproducing systern, a center signal designated by the arrowlabeled C, may he applied equally and in phase to terminals 40 and 46,or to the terminals 42 and 44, or to all four terminals simultaneously,as indicated by the curved ar rows. It will be evident that the C signalwill be subjected to the phase shift of those of networks to which it isapplied, in the example of FIG. 7 to networks 4858, and will become partof the composite signals LT and R1.

The nature of the composite signals appearing at terminals 64 and 66will be seen from the phasor diagrams adjacent the terminals. It is seenthat each of these signals contains a predominant front loudspeakersignal, L and R;', respectively, both of which are shifted in phaserelative to input signals L, and R, by NIH-45). The L -signal furtherincludes signals L and R,,' at to each other, with the signal leading.and in a 45 relationship with L,'. The C signal appears C in bothcomposite signals in the same relative phase position as the signals Iand Rf,

It is significant to note that the R signal contains in addition to thesignal R the two signals L,, and R,,' at 90 to each other. It will beseen, however, that they are reversed in phase relative to the L signal,with R,,' leading and signal I.,,' lagging relative to the correspondingsignals on terminal 64. As noted earlier, however, the signal C is againin the same relative position with respect to the corresponding signalon terminal 64.

Another significant feature of the composite signal is that the L,, andR,, components in one composite signal are in quadratuure with thecorresponding components in the other composite signal, and that the L,component in the L composite signal leads the corresponding component inthe R signal and that the R component in the R signal leads the Rcomponent in the L signal.

Since the signals L, and R, usually will be incoherent signals, ifrecorded on a stereophonic disc record they will appear independently asseparate modulations of the left and right channels. The signals C beingin phase at both terminals 64 and 66 will cause lateral modulation ofthe disc record. The fact that signal L,,' at terminal 64 leads the L,,'signal at terminal 66 by 90 will cause modulation of the record groovein a clockwise advancing spiral, in the manner of a right-hand screwthread. Similarly, because signal R, at terminal 64 lags behind signalR, at terminal 66 by 90 will result in a counter-clockwise helix, in themanner ofa left hand thread. Thus, it is seen that the five signalsapplied to the matrix system of FIG. 7 may be applied to a stereophonicdisc record as five distinct types of modulations, namely, modulation ofthe left and right walls of the groove, lateral modulation, andclockwise and counter-clockwise helical modulation.

The form of modulation on the disc record, as viewed from the point ofview of the cutter tip, looking in the direction of motion of thegroove, is illustrated in FIG. 8. The L, signal causes motion at 45 tothe horizontal. the R, signal causes motion at 45 to the horizontal, andthe C signal causes lateral or horizontal modulation. These threemodulations, it will be recognized, are identical with those whichobtain in the cutting of a conventional stereophonic record. As asignificant departure from conventional practice, there is,additionally, clockwise circular modulation L corresponding to the leftback loudspeaker signal, and counterclockwise circular modulation Rcorresponding to the right back loudspeaker signal. Since the L, and Rmodulations have a significant horizontal component (as projected on theline C) it is evident that they will produce equivalent signalcomponents in the horizontal mode, therefore assuring full compatabilitywith a monophonic phonograph player. An important advantage of thismethod of combining the input signals is that the stereophonic record ortape can be replayed over any stereophonie or monophonic player withfull and complete reproduction of all of the sounds recorded on therecord.

Upon playback, the composite signals L and R-,- depicted by the phasordiagrams of FIG. 7 may be decoded by the decoder described inabove-mentioned US. Pat. No. 3,813,494 to produce four output signalsfor display on four loudspeakers 70, 72, 74 and 76 positioned, asillustrated in FIG. 9, at the left front, right front, left back andright back corners, respectively, of a listening area. Phasor diagramsof the signals appearing on each of these loudspeakers produced by thereferenced decoder are presented adjacent their respective loudspeaker.It will be observed that the signals L,", R,", L," and R predominate inloudspeakers 70, 72, 74 and 76, respectively. The signals from otherchannels appearing in each of the main channels are about 3 dB lower inlevel than the principal signals and, accordingly, tend not to beprominent in the mind of the listener; rather, he will hear primarilythe four independent channels being presented on the four loudspeakers.

While it is important in the encoding process to main tain a 90relationship between the phasors 0707 L,, and 0.707 R,,, it will be seenfrom FIG. 10 that the positions of the phasors L, and R, relative to theL, and R components may be arbitrarily chosen insofar as decoding isconcerned. That is, any decoder designed to decode the composite signalsL and R shown in FIG. 7 will also satisfactorily decode the signals Land R shown in FIG. I0, regardless of the size of the angles a, and 01between phasors L, and 0.707 L and between phasors R, and 0.707 R,,respectively. Since the as (45 in FIG. 7) are established byllJ-l'IfiIWOI'kS 48 and 58, by suitable design of these networks it ispossible to place phasors L; and R, at any desired position with respectto the other two phasors in the group. An especially beneficialrelationship is established by making both the angles or=90 so that inthe phasor group portraying L the phasor L; coincides with phasor 0.707R and in the other phasor group, the phasor R; coincides with phasor0.707 L This relationship is readily obtained by modifyingllI-I'IGIWOI'kS 48 and 58 so that instead of providing a phase-shift(tl1+45) they provide a phase-shift (tl1+90). An important benefit ofthis modification is that the encoding function can be performed withonly four llJ-IIEIIWOI'kS, instead of the six required in the encoder ofFIG. 7.

Referring now to FIG. I], an encoder embodying this improvement has fourinput terminals 80, 81, 82 and 83 to which input signals L], L,,, R, andR, represented by phasors corresponding to the same signals depicted inFIG. 7, are respectively applied. Rather than being applied directly toa Ill-network as in the system of FIG. 7, input terminals and 82 areconnected to a summing junction 84 which is operative to add a unitymeasure of signal L, to 0.707 of signal component R Similarly, terminals81 and 83 are connected to a second summing junction 85 which isoperative to add a unity measure of signal R to 0.707 of signal L,,.Terminal 8] is also connected to the input of a ill-network 86 whichintroduces a relative phase-shift of ll! to the L, signal, and terminal82 is connected to the input of a second (IIH'OQ) network 85. The outputsignals from summing junctions 84 and 85 are respectively applied to theinput terminals of ill-networks 87 and 88, both of which introduce arelative phase-shift of (1l1+90).

The full output of -network 87 is added in summing junction 90 to 0.707of the output of network 86, and similarly, the full output ofill-network 88 is added at summing junction 91 to 0.707 of the output ofill-network 89. As a consequence of this phase-shifting and combining ofsignals, there appears at output terminal 92 a composite signal Ldepicted by phasor group 94, and at output terminal 93 a compositesignal R depicted by phasor group 95. It will be observed that there isa one-to-one correspondence between phasor groups 94 and 95 and thecorresponding phasor groups in FIG. 10 if the angle a in the lattergroup is set at 90.

That the encoder of FIG. 11 is compatible with decoders intended for usewith the signals encoded in accordance with the system of FIG. 7 isdemonstrated by the comparative analysis presented in FIG. I2, asapplied to the decoder described in US Pat. No. 3,784,744. This decoderincludes a pair of input termi- 1 l nals 100 and 102 to which thecomposite signals L-, and R are respectively applied. The signal appliedto terminal 100 is applied to both and is phase-shifted by a pair of II-HCIWOTkS 104 and 106, and the composite R,- signal applied to inputterminal 102 is applied to both of lIJ-IICIWUI'kS 108 and 110. Theseill-networks are of the type previously described, the networks 104 and110 introducing a phase-shift of(tlr'+0) and networks 106 and 108introducing a phase-shift of(tl|+90). It will be noted that thereference angle is designated ill instead of d1, as used in the encoder:this is to call attention to the fact that the reference phase-shift inthe decoder need not be the same as in the encoder, provided the samereference phase-shift is used in all four of lIJ-HEI- works 104, 106,108 and 110. The output signals from networks 104 and 110 are applieddirectly to the leftfront output terminal 112 and to the right-frontoutput terminal 114, respectively. Equal portions of the output signalsfrom networks 106 and 110 are summed in a summing junction 116, theoutput of which is applied to the left-back output terminal 118. andequal portions of the outputs of networks 104 and 108 are summed in asecond summing network 120, the output of which is applied to theright-back output terminal 122.

A comparison will now be made of the performance of the decoder of FIG.12 in response to signals encoded with the encoder of FIG. 7, and tocomposite signals encoded in accordance with the encoder of FIG. 11.Phasor groups corresponding to signals encoded with the encoder of FIG.7 are shown in dotted lines. and the phasor groups encoded by theencoder of FIG. 11 are shown in solid lines. Phasor groups 130 and 132portray the two input signals L and R which, upon being shifted in phaseby the all-pass networks 104, I06, I08 and 110 appear as new phasorgroups 134, I36, 138 and 140. The phasors in these latter four groupsare labeled with a prime to differentiate them from the correspondingphasors prior to introduction of the relative phaseshifts. The signalrepresented by phasor group 134 appears at output terminal 112 as phasorgroup 142 and contains a dominant component L, together with the smallercomponents O.707L,,' and O.707R,,'. The phasor groups 136 and 140 aftersumming in junction 116 result in a signal at output terminal 118represented by phasor group 144 containing a dominant phasor L,,' andsubsidiary phasors 0.707Lf and 0.707R,'. The sum of phasors I34 and 138appearing at the output of summing junction 120 (output terminal 122) isa composite signal represented by phasor group 146 having a dominantphasor R accompanied by subsidiary signals 0.7U7R and 0.707Lf, Finally,the phasor group 140 appears at output terminal 114 as phasor group 148,and contains a dominant signal R together with subsidiary signalsU.707R,,' and (1.7()7L,,'. Thus, the decoded signals appearing at outputterminals 112, 118. 122 and 114 each contains its appropriate dominantsignal together with signals from two other channels reduced inamplitude by the factor 0.707. It will be noted that the two principalfront channel phasors, namely L, in group 142 and R in group 148 are inphase. and that the two principal hack channel vectors, L in phasorgroup 144 and R,,' in group 146, are also in phase with each other. butnot in phase with the L, and R phasors.

It will now be demonstrated that this favorable phase relationship isalso achieved when signals encoded with the encoder of FIG. 11 aredecoded in the decoder of FIG. 12. It will be remembered from thedescription of FIG. 11 that the encoded signals L and R are as portrayedby phasor groups 150 and 152, the former after being acted upon byill-networks 104 and 106 appearing as phasor groups 154 and 156,respectively, and the R signal after transmission throughLIJ-I'ICILWOTkS 108 and 110 appearing as phasor groups 158 and 160,respectively. Phasor group 154 appears at output terminal 112 as phasorgroup 162, and phasor group 160 appears at output terminal 114 as phasorgroup 164. These output signals contain predominant signals L, and Rrespectively, which are in phase with each other, and each includessubsidiary signals 0.707R and O.7U7L,,'.

The phasor groups 156 and 160 upon being summed in summing junction 116produces at output terminal 118 the composite signal portrayed by phasorgroup 166, and the sum of the signals represented by phasor groups 154and 158 appearing at the output terminal 122 of summingjunction 120 isas portrayed by phasor group 168. It will be noted that phasor groups166 and 168 contain predominant phasors L,,' and R respectively, whichare in phase with each other, and also in phase with the predominantphasors in groups 162 and 164, and each accompanied by two subsidiarysignals 0.707R, and 0.707L,'. Comparison of phasor groups 162 and142,166 with 144. 168 with 146, and 164 with 148 reveals that theycontain the same respective subsidiary signals in the same magnitude andin the same intergroup phase relationships. Therefore. the respectivesignals are capable of properly activating the enhancing logic andcontrol circuits described in US. Pat. No. 3,784,744.

Another advantage of the encoder of FIG. 11 will be seen from acomparison of phasor groups 144 and 146, for example, in each of whichthere is shown in dotted line a side signal L which results fromapplying equal amplitude signals to the left-front and left-backterminals of the encoder. Because ofthe angular relationship betweenphasors 0.707L/ and O.707L,. in phasor group 144 as compared to thequadrature relationship between the corresponding phasors in group 166,the resulting phasor L in group 144 is of greater magnitude than thecorresponding phasor in group 166. The absence of exaggeration of the L,signal is of significant advantage, and by symmetry, it will berecognized that exaggeration of an R, signal which would result fromapplication of equal amplitude signals to terminals 82 and 83 of theencoder of FIG. 1] is likewise avoided.

In summary, the encoder of FIG. 1 I offers the following significantadvantages over the encoder of FIG. 7: (l it provides encoding withfour. instead of six III-[18lworks, with an attendant reduction in thecost of the encoder; (2) it produces encoded signals which upondecoding. cause the predominant signals to all be in phase; and (3) itavoids exaggeration of output signal intensity from the decoder whenequal signals are applied to the side terminals of the encoder FIG. 13illustrates a modification of the encoder of FIG. 11, differingtherefrom in the manner in which the four input signals are added andphase-shifted. In this case. the full L; signal applied to terminal 170is added in a summing junction 178 to 0.707 of the R signal applied toinput terminal 174, and the full R; signal applied at input terminal176. is added in summing junction 180 to 0.707 ofthe L signal applied atinput terminal 172. The sum signals from summing junctions I78 and 180are transmitted by respective ill-networks I82 and 184 and are added inrespective summingjunctions I86 and 183 to 0.707 of signals l.,. andR,,. respectively. after being shifted in phase by (QM-90) in respectiveil|networks I90 and 192. The L,-and R signals appearing at outputterminals 194 and 196. represented by phasor groups 198 and 200.respectnely. are similar to the corresponding phasor groups 94 and 95 inFIG. 11 except that in group 198 the 0.707R,, phasor leads the 0.707L,,phasor. whereas in group 94 the L,, phasor leads the R phasor; thepositions of the L,, and R phasors in groups 200 and 95 are similarlyinterchanged. While this decoder provides perfectly consistent signals.it has the slight disadvantage stemming from the fact that the 0.707R,,phasor in group 198 leading the corresponding signal in phasor group 200tends to cause this right-back signal to appear to lean slightly towardthe left-front channel when the record is replayed stereophonically overtwo loudspeakersv By symmetry, the left-back signal likewise will tendto lean slightly to the right when the record is replayedstereophonically. Thus. while the alternative encoder of FIG. I3produces acceptable composite signals for reproduction over fourloudspeakers. it is inferior to the encoder of FIG. 11 if the recordcarrying the encoded sig nals is to be played over a two-channelstereophonic playback apparatus.

By seemingly slight modifications of the encoder of FIG. 13, and of thedecoder shown in FIG. 12 (which is fully described in U.S. Pat. No.3.784.744), the performance ofthe overall system can be significantlyimproved. particularly in its ability to resolve ambiguities in caseswhen the sound signals are "panned"; that is inserted into adjacentchannels in an in-phase relationship. The cause of such ambiguities willbecome apparent from analysis of the decoded signals delivered by thedecoder of FIG. 12 which. it will be remembered. contain predominantsignals L L,,. R, and R respectively. together with two contaminatingsignals from other channels. Actually, the contaminating signals are notnoticed when all four predominant signals are simultaneously present. aswhen four different performers produce four parts of a musical selectionin all four channels. since there is sufficient mixing ofsound in theroom or listening area that the presence ofthe contaminating signals inthe individual channels is inconse quential. They are noticeable,however. when sound is present in only a single channel. or in at mosttwo channels. because in these instances. when the sound should becoming from a single loudspeaker or from two loudspeakers. it is insteadheard from all four. which is readily noticeable and sometimesobjectionable. This situation is improved. and the realism of fourchannel reproduction enhanced by the logic and control systems describedin the aforementioned US. Pat. No. 3.784.744 which recognize thepresence of sounds in individual channels and generate signals forcontrolling the gain of gain control amplifiers in the individualloudspeaker circuits in response to the instantaneous presence of thepredominant signals. Thus. if a signal appears only in the left-frontchannel. for example. (and which. because of the protocol of thedecoder. also appears at reduced level in both of the back chan nels)the logic functions to enhance the gain of the front loudspeakeramplifiers and to turn down the gain ofthe back loudspeaker amplifiersthereby to cause the sound to appear to originate at the left-frontloudspeaker only. The logic and control circuitry operates similarlywith respect to the other three loudspeakers with the consequence thatwhen artists are performing in concert in all four channels the gains ofthe respective amplifiers are increased and decreased to instantaneouslyenhance the channel or channels in which signals are predominant at aparticular instant to give a highly realistic replication of theoriginal four channel program.

The above-described methods of encoding four signals into two anddecoding them back into four works very well in the majority ofcircumstances. one exception, however, being when the sounds are panned.and then only in two specific instances: namely. when the sound ispanned exactly between the two front channels by application of equalamplitude signals to the L, and R, encoder terminals, or when it ispanned precisely between the two back channels L,, and R,,. It can beshown that in these two circumstances the modula tion produced on thestereophonic disc is the same when the two front channels are panned asit is when the two back channels are panned. Consequently. the logic andcontrol system used with the decoder is unable to distinguish whethersuch panned sound signal belongs to the front channels or to the backchannels. resulting in an ambiguity. In accordance with another aspectof this invention. this ambiguity is resolved by another embodiment ofthe encoder and modification of the decoder so to provide a significantimprovement in performance of the system.

The alternative encoder. illustrated in FIG. 14, has four inputterminals 210. 212, 214 and 216 to which the four signals L,. L,,. R,,and R depicted as in-phase signals of equal amplitude. are respectivelyappliedv The total L signal is added in a summing junction 218 to 0.707of the R,, signal. the output of the summing junction being applied to aphase-shifting network 220 which introduces a reference phase-shift 111.The full R signal at terminal 216 is added in summing network 222 to0.707 of the L signal appearing at input terminal 212. and the output ispassed through the ill-network 224, which also provides the referencephaseshift 11/. The L and R,, signals are also applied to respectiveil1networks 226 and 228, each of which provides a phase-shift of Uri-).The full signal appearing at the output of network 220 is added in asumming circuit 230 to 0.707 of the signal appearing at the output ofnetwork 226 to produce at its output terminal 232 a composite signaldesignated L Similarly. the full signal from network 224 is added insumming junction 234 to 0.707 of the signal from network 228, the latterin this case being in the positive sense. The signal appearing at theoutput terminal 236 is the composite signal designated R As in the caseof the other encoders. the signal L and R may be transmitted by FMmultiplex radio, or they may be recorded on any twochannel medium suchas a two-track tape or stereophonic record for later reproduction.

The significance of the modifications to the encoder of FIG. 13 toprovide the encoder of FIG. 14 tnamely. the reversal of the phase of the0.707 terminals of the two summing circuits in the upper half of thediagram) will be appreciated from an analysis of the phasor rela'tionship of the L and R composite signals portrayed as phasor groups 238and 240. respectively. It will be observed that phasor group 238consists of the signal L;(which although shown in the same phaserelationship as the input signal L, has a tfl-asa-function-offrequencyangle difference between them), a signal 0.707R in a negative sense withrespect to its corresponding input phasor, and a 0.707L,, signal whichlags phasor 0.707R,, by 90 because of the action of network 226. Phasorgroup 240 consists of the original signal R, in the same relative phaseposition as its corresponding input signal, a signal 0.707L in phasewith the R; signal, and a 0.707R signal lagging the .707L,, signal by 90due to the action of ill-network 228. As has been pointed outhereinabove, in the interest of providing better realism of imageplacement when the record is played in conventional stereophonic modeover two loudspeakers, it is preferable to arrange the phasor 0.707L inphasor group 240 to lag behind the corresponding phasor in phasor group238, and conversely, to arrange the phasor 0.707R,, in phasor group 238to lag behind the corresponding phasor in group 240. Alternatively, thepositions of phasor groups having its component phasors oriented asdepicted at 238 and 240 can be interchanged and still enjoy the benefitsof the invention. For example, simply by reversing the order in whichthe input signals are applied to the en coder-that is, R; to terminal210, R,, to terminal 212, L b to terminal 214 and L, to terminal 2I6-thecomposite signal R having the components shown in pa rentheses in phasorgroup 238 will appear at output ter minal 232 and the composite signal Lhaving the components shown in parenthesis in phasor group 240 willappear at output terminal 236.

Referring now to FIG. 16, the effect of panning is to divide the signal(as by means of two coupled attenuators) between two channel inputs. Atthe mid-point of the panned operation, the signal becomes divided evenlybetween the front channels L, and R,, or between the back channels L orR this condition will now be examined. The phasor groups 238 and 240from FIG. 14 are repeated here as phasor groups 250 and 252,respectively, and the panned center signals have been added. The frontcenter signal, C,, is placed in the proportion 0.707C, and in-phase inthe phasor groups 250 and 252, appearing as phasors 254 and 256. Sincethese phasors are equal and in-phase the signals L and R combine toproduce a horizontal motion of the cutter stylus; accordingly, thecenter front signal C, appears as a horizontal arrow 246 in FIG. 15.From the discussion thus far, it is seen that FIG. I5 depicts the leftfront channel phasor, Ly, the center channel phasor, C;, and theright-front channel phasor, R in a relationship which those skilled inthe art will recognize as portraying the modulation of a conventionalstereophonic record.

Reverting now to FIG. 16, it will be noted that the center-back signal,C,,, is divided in the proportion 0.707 in the left back and right backchannels, and since these two phasors already appear as a 0.707fraction, the corresponding fraction of the C signal is 0.5 in phasewith the 0.707L,, phasor and 0.5 in phase with the 0.707R phasors inboth phasor groups. With this convention in mind, it is seen that thetwo phasors in each group add to the larger phasors 0.707C,, in each ofthe phasor groups 250 and 252; however, it should also be observed thatthe phasor 0.707C,, in phasor group 250 is out-of-phase with thecorresponding phasor in group 252. This is an important quality of theencoder of FIG. I4 because now the center back signal,

C,,, is of an entirely different character than the center front signalC,. It will be recognized that the signal, C having an out-of-phaserelationship in the two channels will result in a vertical modulation ofthe record groove, which is depicted by the arrow 248 in FIG. 15. Itwill be realized that any signal recorded in this man' ner cannot bereproduced by a monophonic phonograph pick-up, nor by the monophonicsection of an FM multiplex transmitting station; consequently, whenusing the encoder of FIG. I4 the centerback location should preferablybe used for occasional sounds such as reverberation, motion duringpanning, etc., and not for the placement of an important artist, becausehe would not be heard when the signal is broadcast over AM radio or overmonophonic FM radio. Such signals would, however, be fully audible withstereophonic or quadraphonic modes or reproduction, and all otherlocations of the artist would be transmitted satisfactorily.

Another significant feature of the FIG. 14 encoder is illustrated by thephasor groups 256 and 258 in FIG. 17, the former depicting the situationwhich results when the phasor groups 250 and 252 of FIG. 16 are addedand the latter depicting the situation when the composite signal R(phasor group 252) is subtracted from L (phasor group 250). It will benoted that when L and R are added the phasors L], L,,, R,, and R, allhave an amplitude equal to unity, whereas the front center signal, Cy,is augmented by a factor l.4l4, which is exactly what happens when astereophonic record is played over a monophonic player. The back centersignal, C, is cancelled, however, because of the aforementionedout-of-phase relationship. When the phasor groups are subtracted, thephasors L L R,, and R, again all appear with unity amplitude, but thistime the center back signal, C is augmented by the factor I.4l4 whilethe center front signal, C is canceled. The relationship portrayed byphasor groups 256 and 258 are extremely important since they indicatethat if only a center front signal is present (i.e., no center backsignal) the phasor group 256 will be greater than group 258, and,conversely, if there is only a center back signal but no center frontsignal, the phasor group 258 will be larger. This interesting propertyis used to advantage to enhance the operation of the decoder to beutilized with the encoder of FIG. 14, all as fully described in Us. Pat.No. 3,821,471.

Additional features and attributes of the abovedescribed encoders willnow be described in connection with FIG. I8 which illustrates theencoder of FIG. 14 modified as described above to produce the compositesignals L R having the components shown in parentheses in phasor groups238 and 240 of FIG. 14. More specifically, in the encoder of FIG. 18 thefull L, signal is added in a summing junction 326 to 0.707 of the R,,signal, the output signal from the summing junction 326 being applied toan all-pass phase-shifting network 328 which introduces a referencephase-shift III which varies as a function of frequency. The full Rsignal at terminal 316 is added in a second summing junction 330 to0.707 of the L,, signal appearing at input terminal 3I2, and the sumsignal is passed through a second Ill-network 332 which also providesthe reference phase-shift ill. The L,, and R,, signals are applied torespective Ill-networks 334 and 336, each of which pro vides a phaseshift of (lb-) and wherein the IIJ-fLIIIC- tions are essentially thesame. The full signal appearing at the output terminal of network 328 isadded in a

1. Apparatus for transforming a multi-channel program including at least first, second, third and fourth program signals into two composite signals, said apparatus comprising: first, second, third and fourth input terminals to which said first, second, third and fourth signals to the extent they are present are respectively applied, first and second output terminals, means connected between said first and fourth input terminals and said first and second output terminals, respectively, operative to transfer substantially equal amplitude proportions of said first and fourth signals to said first and second output terminals, respectively, and means connected between both said second and third input terminals and both said first and second output terminals operative to transfer substantially equal reduced amplitude proportions of said second and third signals to both said first and second output terminals and including phase-shifting networks operative to provide a substantially constant differential phase-shift angle between said second and third signals at one of said output terminals and said second and third signals, respectively, at the other of said output terminals over the frequency range of interest.
 2. Apparatus in accordance with claim 1 wherein said first-mentioned means is operative to transfer said first and fourth signals to said first and second output terminals, respectively, with the same relative phase relationship they exhibit at their respective input terminals.
 3. Apparatus in accordance with claim 1 wherein said differential phase-shift angle has a value of substantially 90* and said second signal at one of said output terminals leads said second signal at the other of said output terminals and said third signal at said one output terminal lags said third signal at said other output terminal.
 4. Apparatus in accordance with claim 1 wherein the proportions of said second and third signals transferred to said output terminals are substantially 3db down in amplitude from the proportions of said first and fourth signals transferred to said output terminals.
 5. Apparatus in accordance with claim 1 wherein said second signal at one of said output terminals leads said second signal at the other of said output terminals and said third signal at said one output terminal lags said third signal at said other output terminal.
 6. Apparatus for transforming a multi-channel program including two or more signals identified as Lf, Rf, Lb and Rb into two composite signals, said apparatus comprising: first, second, third and fourth input terminals to which said Lf, Lb, Rb and Rf signals to the extent they are present are respectively applied, first and second output terminals, means connected between said first and fourth input terminals and said first and second output terminals, respectively, operative to transfer said Lf signal to said first output terminal and to transfer said Rf signal to said second output terminal, and means connected between both said second and third input terminals and both said first and second output terminals operative to transfer reduced amplitude proportions of said Lb and Rb signals to both of said output terminals with a 90* relative phase shift angle and to cause said Lb signal at one of said output terminals to lead said Lb signal at the other of said output terminals and said Rb signal at said one output terminal to lag said Rb signal at said other output terminal.
 7. Apparatus according to claim 6 wherein said means connected between said input terminals and said output terminals includes first and second summing circuits each having first, second and third output terminals and an output terminal, the output terminal of said first and second summinG circuits being connected to said first and second output terminals, respectively, each of said summing circuits being operative to produce at its output terminal a composite signal including predetermined proportions of signals applied to its first, second and third input terminals, first and second all-pass phase-shifting networks each operative to shift the relative phase of signals applied thereto by an angle differing from a reference phase angle by 45*, said first and second phase-shifting networks being respectively connected between said first input terminal and the second input terminal of said first summing circuit and between said fourth input terminal and the second input terminal of said second summing circuit, third and fourth all-pass phase-shifting networks each operative to shift the relative phase of signals applied thereto by an angle differing from the aforesaid reference phase angle by 90*, said third and fourth phase-shifting networks being respectively connected between said second input terminal and the first input terminal of said second summing circuit and between said third input terminal and the first input terminal of said first summing circuit, and fifth and sixth all-pass phase-shifting networks each operative to shift the relative phase of signals applied thereto by the aforesaid reference phase angle, said fifth and sixth phase-shifting networks being respectively connected between said second input terminal and the third input terminal of said first summing circuit and between said third input terminal and the third input terminal of said second summing circuit.
 8. Apparatus according to claim 7 wherein said first and second summing circuits are each operative to produce at its output terminal a composite signal including a first signal corresponding in amplitude to a signal applied to its second input terminal and second and third signals each having an amplitude corresponding to 0.707 of the amplitude of signals applied to its first and third input terminals.
 9. Apparatus for transforming a multi-channel program including two or more signals identified as Lf, Rf, Lb and Rb intended for reproduction by loudspeakers positioned at the left front, right front, left back and right back corners, respectively, of a listening area into two composite signals, said apparatus comprising, first, second, third and fourth input terminals to which said Lf, Lb, Rb and Rf signals to the extent they are present are respectively applied, first and second output terminals, means connected between said first and fourth input terminals and said first and second output terminals, respectively, operative to transfer substantially equal amplitude proportions of said Lf and Rf signals to said first and second output terminals, respectively, with the same relative phase relationship they exhibit at their respective input terminals, and means connected between both said second and third input terminals and both said first and second output terminals operative to transfer substantially equal reduced amplitude proportions of said Lb and Rb signals to both of said output terminals with a substantially 90* phase-shift angle between said Lb signals and between said Rb signals at said output terminals and to cause the Lb signal at one of said output terminals to lead the Lb signal at the other of said output terminals and to cause the Rb signal at said one output terminal to lag the Rb signal at said other output terminal.
 10. Apparatus in accordance with claim 9 wherein the proportion of said Lf and Rf signals transferred to said output terminals is related to the proportion of said Lb and Rb signals transferred to said output terminals by substantially the ratio 1:0.707.
 11. Apparatus in accoRdance with claim 10 wherein substantially the full Lf and Rf signals applied to said first and fourth input terminals are transferred to said first and second output terminals, respectively.
 12. Apparatus for transforming a multi-channel program including up to four audio information signals identified as Lf, Lb, Rb and Rf into two composite signals, said apparatus comprising: first, second, third and fourth input terminals to which the Lf, Lb, Rb and Rf signals, to the extent they are present, are respectively applied, first and second output terminals, a first summing circuit connected to add the Lf signal from the first input terminal to a reduced amplitude proportion of the Rb signal from the third input terminal, a second summing circuit connected to add the Rf signal from the fourth input terminal to a reduced amplitude proportion of the Lb signal from the second input terminal, a third summing network connected to add the signal from the first summing network to a reduced amplitude proportion of the Lb signal from the second input terminal and first phase-shifting means operative to cause the signal received by said third summing network from the second input terminal to be substantially in phase quadrature with the signal received from said first summing network, a fourth summing network connected to add the signal from the second summing network to a reduced amplitude proportion of the Rb signal from the third input terminal and second phase-shifting means operative to cause the signal received by said fourth summing network from the third input terminal to be substantially in phase quadrature with the signal received from said second summing network, and means connecting the output terminals of said third and fourth summing networks to said first and second output terminals, respectively.
 13. Apparatus in accordance with claim 12 wherein said first and second summing circuits are operative to add the signals from the first and third input terminals and from the fourth and second input terminals, respectively, substantially in the ratio 1: 0.707.
 14. Apparatus in accordance with claim 13 wherein said third and fourth summing networks are operative to add the signals from the first summing network and the second input terminal and the signals from the second summing network and the third input terminal, respectively, substantially in the ratio 1:0.707.
 15. Apparatus in accordance with claim 13 wherein said first summing network is operative to add the signals from the first and third input terminals substantially in the ratio 1:-0.707, and the second summing network is operative to add the signals from the fourth and second input terminals substantially in the ratio 1:0.707.
 16. Apparatus in accordance with claim 15 wherein said third summing network is operative to add the signals from the first summing network and the second input terminal substantially in the ratio 1:-0.707 and the fourth summing network is operative to add the signals from the second summing network and the third input terminal substantially in the ratio 1:0.707.
 17. Apparatus for transforming a multi-channel program including up to four audio information signals identified as Lf, Rf, Lb and Rb into two composite signals, said apparatus comprising: first, second, third and fourth input terminals to which said Lf, Lb, Rb and Rf signals to the extent they are present are respectively applied, first and second output terminals, and signal transfer means connected in circuit between said input terminals and said output terminals for transferring signals from said input terminals to said output terminals, said signal transfer means including first, second, third and fourth summing circuitS each having first and second input terminals and an output terminal, means connecting said third and first input terminals to the first and second terminals, respectively, of said first summing circuit, said first summing circuit being operative to add unequal portions of said Lf and Rb signals, means connecting said second and fourth input terminals to the first and second input terminals, respectively, of said second summing circuit, said second summing circuit being operative to add unequal portions of said Lb and Rf signals, first and second like all-pass phase-shifting networks connected between the output terminal of said first summing circuit and the first input terminal of said third summing network and between the output terminal of said second summing circuit and the first input terminal of said fourth summing circuit, respectively, third and fourth like all-pass phase-shifting networks connected between said second and third input terminals and the second input terminal of said third and fourth summing circuits, respectively, said third and fourth phase-shifting networks being operative to cause a phase-shift to signals applied thereto differing by 90* to the phase-shift introduced by said first and second phase-shifting networks, said third and fourth summing circuits being operative to add unequal portions of signals applied to their first and second input terminals, and means connecting the output terminals of said third and fourth summing circuits to said first and second output terminals, respectively.
 18. Apparatus according to claim 12 wherein said first and second phase-shifting networks are operative to shift the relative phase of signals applied thereto by a predetermined reference angle and said third and fourth phase-shifting networks are operative to shift the relative phase of signals applied thereto by an angle differing from said reference angle by 90*.
 19. Apparatus according to claim 18 wherein said first and second summing circuits are each operative to add 0.707 of a signal applied to its first input terminal to 1.00 of a signal applied to its second input terminal.
 20. Apparatus according to claim 19 wherein said third and fourth summing circuits are each operative to add 1.00 of a signal applied to its first input terminal to 0.707 of a signal applied to its second input terminal, whereby in the composite signal appearing at said first output terminal said Lf signal is in phase with the 0.707 Rb signal component thereof, and in the composite signal appearing at said second output terminal said Rf signal is in phase with the 0.707 Lb signal component thereof.
 21. Apparatus according to claim 18 wherein said first summing circuit is operative to add -0.707 of a signal applied to its first input terminal 1.00 of a signal applied to its second input terminal, said second summing circuit is operative to add 0.707 of a signal applied to its first input terminal to 1.00 of a signal applied to its second input termrinal, said third summing circuit is operative to add 1.00 of a signal applied to its first input terminal to -0.707 of a signal applied to its second input terminal, and said fourth summing circuit is operative to add 1.00 of a signal applied to its first input terminal to 0.707 of a signal applied to its second input terminal, whereby in the composite signal appearing at said first output terminal said Lf signal is in phase opposition with the 0.707 Rb signal component thereof, and in the composite signal appearing at said second output terminal said Rf signal is in phase with the 0.707 Lb signal component thereof.
 22. Apparatus according to claim 17 wherein said third and fourth phase-shifting networks are each operative to shift the relative phase of signals applied Thereto by a predetermined reference angle and said first and second phase-shifting networks are each operative to shift the relative phase of signals applied thereto by an angle differing from said reference angle by 90*.
 23. Apparatus according to claim 22 wherein each of said first and second summing circuits is operative to add 0.707 of a signal applied to its first input terminal to 1.00 of a signal applied to its second input terminal, and wherein each of said third and fourth summing circuits is operative to add 1.00 of a signal applied to its first input terminal to 0.707 of a signal applied to its second input terminal.
 24. A method for transforming a multi-channel program including at least first, second, third and fourth audio information signals, to the extent they are present, into two composite signals suitable for recording or transmitting on only first and second channels, comprising the steps of: transferring substantially equal predetermined-amplitude proportions of said first and fourth signals to said first and second channels, respectively, and transferring substantially equal predetermined smaller amplitude proportions of both said second and third signals to both said first and second channels with a substantially constant differential phase-shift angle between said second signal in said first channel and said second signal in said second channel, and between said third signal in said first channel and said third signal in said second channel, over the audio frequency range of interest.
 25. A method for transforming a multi-channel program including up to four audio information signals identified as Lf, Lb, Rb and Rf, if present, into first and second composite signals identified as LT and RT, respectively, suitable for recording or transmitting on first and second channels, respectively, comprising the steps of: transferring substantially equal amplitude proportions of said Lf and Rf signals to said first and second channels, respectively, and transferring substantially equal smaller amplitude proportions of both said Lb and Rb signals to both said first and second channels with a substantially constant differential phase-shift angle of substantially 90* between said Lb and Rb signals in said first channel and said Lb and Rb signals, respectively, in said second channel over the audio frequency range of interest.
 26. A method according to claim 25 wherein the transferred proportion of said Lf and Rf signals is related to the transferred proportion of said Lb and Rb signals by substantially the ratio 1:0.707.
 27. A method for transforming a multi-channel program including up to four audio information signals identified as Lf, Lb, Rb and Rf, to the extent they are present, into first and second composite signals identified as LT and RT, respectively, suitable for recording on first and second channels, respectively, comprising the steps of: adding the Lf signal to a reduced amplitude proportion of the Rb signal to produce a first sum signal, adding the Rf signal to a reduced amplitude proportion of the Lb signal to produce a second sum signal, adding said first sum signal to a reduced amplitude proportion of the Lb signal shifted in phase relative to said first sum signal by substantially 90* to produce a first composite signal LT containing the Lf signal and substantially equal reduced amplitude proportions of the Lb and Rb signals, and adding said second sum signal to a reduced amplitude proportion of the Rb signal shifted in phase relative to said second sum signal by substantially 90* to produce a second composite signal RT contAining the Rf signal and substantially equal reduced amplitude proportions of the Lb and Rb signals.
 28. The method according to claim 27 wherein said reduced amplitude Lb and Rb signals are relatively phase-shifted to cause the Lb signals in the two composite signals to be substantially in phase quadrature and the Rb signals in the two composite signals to also be substantially in phase quadrature and the Lb and Rb signals in one of the two composite signals to lead and lag, respectively, the Lb and Rb signals in the other composite signal, said Lf signal to be either in phase or in phase opposition with the Rb signal in said first composite signal and the Rf signal to be either in phase or in phase opposition with the Lb signal in said second composite signal, and at least one of the Lf and Rf signals in said composite signals to be in phase with the signal with which it is either in phase or phase opposition.
 29. The method according to claim 28 wherein the Lf signal in said first composite signal is in phase with the Rb signal in said first composite signal.
 30. The method according to claim 28 wherein the Lf signal in said first composite signal is in phase opposition with the Rb signal in said first composite signal.
 31. The method according to claim 30 wherein the Rf signal in said second composite signal is in phase with the Lb signal in said second composite signal.
 32. Apparatus for transforming a multi-channel program including up to four audio information signals identified as Lf, Lb, Rb and Rf into two composite signals suitable for recording or transmitting on respective separate channels, said apparatus comprising; first, second, third and fourth input terminals to which the Lf, Lb, Rb and Rf signals to the extent they are present are respectively applied, first and second output terminals, signal-transfer means including a plurality of summing networks and a plurality of phase-shifting networks connected in circuit between said second and third input terminals and said output terminals, said signal-transfer means being operative to transfer substantially equal portions of the Lb and Rb signals to both of the output terminals with the Lb signals at the two output terminals substantially in phase quadrature and with the Rb signals at the two output terminals also substantially in phase quadrature and the Lb and Rb signals at one of the output terminals leading and lagging, respectively, the Lb and Rb signals at the other output terminal, and means directing connecting said first and fourth input terminals to said first and second output terminals, respectively, for transferring the Lf and Rf signals to the first and second output terminals, respectively, with the same relative phase relationship that they exhibit at their respective input terminals, said phase-shifting networks being operative to shift the Lb and Rb signals transferred therethrough by phase-shift angles which include a reference angle which varies as a function of frequency, thereby causing said Lf and Rf signals to be equally displaced relative to their associated quadrature-related Lb and Rb signals by said reference angle.
 33. Apparatus for transforming a multi-channel program including up to four audio information signals indentified as Lf, Lb, Rb and Rf into two composite signals suitable for recording or transmission on respective separate channels, said apparatus comprising, in combination: first, second third and fourth input terminals to which the Lf, Lb, Rb and Rf signals to the extent they are present are respectively applied, first and second output terminals, and signal-transfer means connected between said input terminals and said output terminals for combining the signals applied to the input terminals with predetermined amplitude and phase relationships and transferring them to the output terminals, said signal-transfer means including first, second, third and fourth phase-shifting networks each of which is operative to shift the phase of signals applied thereto by a frequency-dependent reference angle, and said second and third phase-shifting networks each being operative to introduce an additional phase shift of substantially 90*, means including first summing means for coupling the sum of said Lf signal and a portion of said Lb signal through said second phase-shifting network to said first output terminal, means including second summing means for coupling the sum of said Rf signal and a portion of said Rb signal through said third phase-shifting network to said second output terminal, means including said fourth phase-shifting network for coupling a portion of said Lb signal to said second output terminal, and means including said first phase-shifting network for coupling a portion of said Rb signal to said first output terminal, whereby said signal-transfer means is operative to transfer substantially equal portions of the Lb and Rb signals to both of the output terminals with the Lb signals at the two output terminals substantially in phase quadrature and with the Rb signals at the two output terminals also substantially in phase quadrature, and with the Lb and Rb signals at one of the output terminals leading and lagging, respectively, the Lb and Rb signals at the other output terminals, and to transfer the Lf signal to the first output terminal in phase with the Lb signal at said first terminal and to transfer the Rf signal to the second output terminal in phase with the Rb signal at said second output terminal and in anti-phase relationship with the Lf signal at said first output terminal. 